Ion mobility spectrometry is an analytical technique used in chemistry to analyze and detect a wide variety of chemical compounds. Ion mobility spectrometry has been found to be particularly advantageous for detecting trace amounts of organic molecules. Ion mobility spectrometry is also known by the name plasma chromatography.
The first step of ion mobility spectrometry is to take a sample and pass it through an ionizer to produce electrically charged molecules usually called ions. These ions are allowed to enter a drift region provided with an electric field which causes the charged molecules to move through the drift region. An electrical gate is provided to control the entrance of ions at one end of the drift region. An ion detector is provided at or near the opposite end of the drift region to detect ions which have traversed therethrough.
The drift region is provided with a steady flow of relatively inert drift gas flowing opposite to the direction that the ions travel under the force of the electric field. The time which it takes for ions to travel from the entrance gate across the drift region depends upon the mass, geometry and size of the molecule, its electrical charge, and the distance which must be traveled. Other factors may also effect the velocity or mobility of the ions in the drift region to produce varying transit times which are characteristic or indicative of the types of molecules present.
Ion mobility spectrometers can be used as detectors for gas chromatographs or to further analyze the outflow from a gas chromatograph to provide data useful in identifying the constitutents of the sample being analyzed. One popular type of gas chromatograph incorporates a long, relatively small diameter capillary tube which is either coated or uncoated. The sample gas is heated and introduced into the capillary tube. The constituents of the sample proceed through the tube swept along by a very small flow of helium or other appropriate carrier gas. Different molecules pass through the capillary tube in differing amounts of time, thus allowing the sample to be segregated into different constituents which exit the capillary over a range of times. The constituents exit the capillary tube and are detected by some form of chromatographic detector.
It has been found that ion mobility spectrometers can advantageously be used as chromatographic detectors to analyze the outflow from a gas chromatograph to provide ultrasensitive detection of organic and other compounds contained therein. Unfortunately, prior methods for using ion mobility spectrometers as detectors for gas chromatographs have not proven satisfactory in many applications and have not gained wide acceptance.
Presently there are three methods used in ion mobility spectrometry. The first is the single scan method which involves opening the entrance gate for a short period of time to admit ions into the drift region. The entrance gate is typically left open for approximately 0.2 millisecond before it is closed thereby blocking further ions from entering the drift region. The pulse of ions admitted during the period the entrance gate is open move down the drift region under the force of the electric field countercurrent to the drift gas. In the single scan method there is no exit gate and the ions strike directly onto the ion detector. The ion detector is connected to an oscilloscope which is used to directly monitor the output of the ion detector. The single scan method suffers from high noise levels in the detector output signal and accordingly cannot be used for high resolution chromatographic separations.
The second method used in ion mobility spectrometry is termed the signal averaging method. The signal averaging method again involves opening the entrance gate to allow a pulse of ions to pass into the drift region and be propelled therethrough under the force of the electric field. The ions again strike directly upon the ion detector, as in the single scan method. In the signal averaging method, many repetitions of single scans are performed and recorded such as with a computer. Five hundred to 1,000 repetitions are often required to provide statistically acceptable signal to noise levels. Such numerous repetitions typically require a minimum of 10 to 20 seconds to perform thereby rendering the method unacceptable for high resolution chromatographic separation of sample constituents on an on-line basis as they flow directly from a capillary or other gas chromatograph.
The third method currently used in ion mobility spectrometry is termed the moving second gate method. Such method is used on ion mobility spectrometers having an exit gate which is located at the end of the drift region prior to the ion detector. A pulse of ions are admitted through the entrance gate and propelled through the drift region under the force of the electric field. The second or exit gate positioned at the end of the drift region selectively opens for a short period of time usually equal to the time the entrance gate is opened. The delay time between opening of the entrance and opening of the exit gate is the control variable which is varied over a range of delay times. The time delay between opening of the entrance gate and subsequent opening of the exit gate thus allows only ions having transit times approximately equal to the delay to pass through the exit gate and onto the ion detector where the strength of the measured signal indicates the quantity of ions having such transit time.
The time delay between the opening of the entrance gate and the opening of the exit gate is varied over a range of relevant transit times and a large number of experimental ion pulses are needed in order to generate acceptable data indicating ion quantity (signal strength) as a function of ion transit time. Because of the large number of different delay times which must be tested, the amount of time necessary to test the full spectrum of transit times with sufficient specificity, accuracy, and to obtain acceptable signal to noise levels thus requires testing for a minimum of one or two minutes. Thus the moving second gate method is also not acceptable for analyzing constituents as they flow directly from a gas chromatograph.
None of the three methods described above make efficient use of the available ions and this is particularly true with the moving second gate method. Typical entrance gate pulse durations of 0.2 milliseconds with recurrent pulses every 20 milliseconds allow only one percent of the available ions from the sample to pass into the drift region. This small proportion of the available ions admitted into the drift region is further reduced in the moving second gate method by the selective opening and closing of the exit gate for approximately similar periods of time. This results in an average of only one percent of the ions admitted through the entrance gate passing through the exit gate and onto the ion detector. The resulting 0.01 percent utilization of available ions allows only a relatively weak signal to be developed at the ion detector, thus causing a poor signal to noise ratio. Opening the entrance and exit gates for longer periods of time increases the signal strength but lowers the resolution of the spectrometer. Resolution is reduced because the available time between opening of the entrance gate and closing of the exit gate is greater and the range of successful transit times also increases. This tends to blur the resulting detector signal and detracts from the acceptability of the method for determining sample constituents.
In any method a short duration pulse of ions entering through the entrance gate provides greater resolution but is relatively weak because of the small number of ions available to strike the detector. The resulting low level ion detector signals cause the signal to noise ratio to become unacceptably high for producing statistically accurate results unless a very large number of ion pulses are tested. Increasing the number of ion pulses tested requires greater amounts of time for performing an accurate analysis of the sample. Such increased analysis time renders the method unacceptable for use as an on-line detector for capillary gas chromatographs and in other types of chromatographs where the chromatographic eluants are admitted from the chromatograph over periods of time usually ranging from 1-5 seconds in duration.